1. Field of the Invention
The present invention relates to a light receiving device for receiving light, and particularly to an avalanche photodiode that is excellent in device characteristics such as reliability and can be manufactured in a high yield.
2. Description of the Related Art
Avalanche photodiodes (hereinafter referred to as APDs) are excellent in device characteristics and may be manufactured in a high yield, therefore they have been applied to light receiving devices in optical-fiber communication systems, etc. Regarding the structures of these APDs, two types of structures, mesa-type and planar-type are known.
The mesa-type APD has a structure in which a depletion region of semiconductor layers such as a multiplication layer, an electric field-buffer layer, and a light absorption layer that are formed on an substrate is exposed to the surface. In detail, for example, a mesa-type APD disclosed in Japanese Laid-Open Patent Publication 312,442/1995 (Patent Document 1) has each of crystalline layers such as a p+ type InP buffer layer, p− type InGaAs light absorption layer, a p+ type InP field buffer layer, an n− type InAlGaAs/InAlAs super-lattice multiplication layer, an n+ type InAlAs cap layer, and an n+ type InGaAs contact layer that are grown on a p+ type InP substrate. Then, a p+ type conductive region is formed so as to surround a light receiving region that is formed at the center portion of the n+ type InAlAs cap layer and the n+ type InGaAs contact layer, and a ring-shaped groove that has a depth reaching the super-lattice multiplication layer is formed between the light receiving region and the p+ type conductive region. In such a mesa type APD, a flip-chip-type electrode structure is mainly used, in which an electrode of another device is directly connected to the circular n-type electrode formed on the light receiving region, and detection light is incident from the back face of the APD.
On the other hand, the planar-type APD is structured in such a way that the depletion region of semiconductor layers such as a multiplication layer, an electric field-buffer layer, and a light absorption layer is not exposed to the surface, meanwhile a stable InP planar p-n junction, etc. is exposed to the surface. Because planar-type APDs are excellent in manufacturing and handling, recently they have been widely applied to optical communication fields, and their various improvements have been made.
For example, a conventional planar-type APD disclosed in Japanese Laid-Open Patent Publication 330,530/1999 (Patent Document 2) has a laminated structure composed of specific six layers on a semiconductor substrate, a specific conductive region provided on the outer circumference of a light receiving region, and a specific circular ring-shaped separation-groove region in the light receiving region. Moreover, in order to further decrease equipotential-line concentration at the light receiving edge, a circular ring-shaped region that is thinner than the thickness of a semiconductor multiplication layer is formed in a position inscribed in the circular ring-shaped separation groove provided on the outer circumference of the light receiving region in a second conductive-type semiconductor cap layer.
In each of the columnar n-type conductive layers, in order to prevent tunnel current flowing, a semiconductor layer composed of crystalline material having wider band gap is needed. Moreover, in order to reduce device resistance, the layer needs to be thicker than a certain value. Therefore, the depth of the ring-shaped groove is made relatively deep comparing to the thickness of an SiNx surface passivation layer or an n-type electrode. As a result, in the ring-shaped groove, especially at the edge of the groove, the thickness of the surface passivation layer becomes extremely thin or lost, that is, discontinuity occurs; thereby, protection of the covered portion becomes not enough, causing a problem in which device characteristics such as electrical characteristics and device reliability deteriorates.
Here, in an APD disclosed in Patent Document 2, a circular ring-shaped region is provided on the side wall of the light receiving region, as a result, it has a structure as provided with a step; however, because the step height is set to be as low as not more than the thickness of an avalanche multiplication layer, this method is not effective enough with respect to the discontinuity in the surface passivation layer, etc. occurring at the edge of the ring-shaped groove.
Also, in the device manufacturing process, when a resist, an SiNx film, or an SiOx film, etc. is used for a mask for performing various kinds of processes such as etching, the discontinuity at the edge of the ring-shaped groove occurs. As a result, such troubles as etching solution penetrates into a covered portion occurs during device fabrication; consequently, which has caused such problems as the device characteristics are deteriorated or a manufacturing yield is decreased.
Also, in the APD having the structure in which light is incident from the back face as disclosed in Patent Document 1, positional alignment to make light incident on the light receiving region is generally difficult, therefore it is preferable, from a viewpoint of operation, to adopt a structure in which light is incident on the surface. However, in the structure in which light is incident on the surface, because light is incident on it from the side of n-type conductive layers each surrounded by the ring-shaped groove, an external circuit cannot be provided on the light receiving region; consequently, an external electrode for connecting the region to the external circuit needs to be provided outside the light receiving region, that is, outside the ring-shaped groove. Therefore, in order to electrically connect the n-type electrode provided on the light receiving region inside the ring-shaped groove with the external electrode provided outside the ring-shaped groove, a lead needs to be provided for electrically connecting both the electrodes across over the ring-shaped groove. Accordingly, in the ring-shaped groove, especially at the edge of the groove, so-called discontinuity occurs in which the film thickness of the lead becomes extremely thin or the lead is broken; thereby, the electric characteristics has been deteriorated, causing a problem in which device characteristics such as device reliability worsens.